Plasmas, and method for producing them and using them, have been known for some time. In many applications of plasmas, it is desirable to confine the very high-temperature plasma to as small a volume as possible for as long as possible by means other than mechanical walls, despite its strong tendency to expand and to cool, due to a number of known cooling effects. If a plasma is produced within an ordinary container, without special provisions for confinement, its temperature will rapidly decrease because of plasma expansion, nuclear reradiation, plasma instabilities, bremmsstrahlung radiation from heavy-element contaminants, and the transfer of energy from the nuclei to the walls of the container by collisions. If, instead, a high-temperature plasma of a given density can be appropriately confined for a sufficient time, without contacting heat-absorbing walls, nuclear fusion and production of neutrons will be enhanced. The desired confinement time for a given type of plasma at a given temperature t is inversely related to the number n of electrons per unit volume (the "particle density"), and hence it is the product nt which one wishes to maximize.
It is known that strong magnetic fields, applied in any of a number of different geometries, have a tendency to confine the hot plasma. However, although the thermal pressure of the plasma (for example, for a 10 Kev plasma of a mixture of deuterium and tritium of suitable density) is not greater than the effective pressure exerted on the plasma by presently attainable magnetic fields, the lack of a stable equilibrium causes rapid plasma losses. So far as is known, the best laboratory results using such magnetic fields alone have been to confine plasmas with particle densities of 10.sup.13 to 10.sup.14 per cubic centimeter for times of 10.sup.-2 to 10.sup.-1 second or with densities of from 10.sup.15 to 10.sup.16 per c.c. for times of the order 10.sup.-6 second, or with densities of the order of 10.sup.21 per c.c. for times of the order of 10.sup.-9 second, giving values of nt orders of magntidue lower than would be desirable in many applications.
The important conditions to be met for the attainment of intense neutron generation, and useful nuclear fusion energy production, are then (1) production of a high-temperature plasma of a suitable material; (2) confinement of the plasma to a volume sufficiently small to minimize energy losses to the chamber wall; (3) supplying sufficient energy after initiation of the reaction to overcome energy losses due to re-radiation, instabilities and bremsstrahlung radiation from contaminants; and (4) maintaining the high temperature and confinement for a sufficient length of time.
Neutron radiations have been found especially useful for their sterilizing effect in killing harmful bacteria and other microorganisms, and have substantial utility for such purposes. Nuclear fusion is of course useful in nuclear experimentation and in the production of usable energy. References relating to use of neutrons in food sterilization include: paper entitled "Ionizing Radiation in Processing Meat and Dairy Products", by C. F. Niven, Jr., presented at a meeting held at Michigan State University, East Lansing, Michigan, Jan. 12-14, 1956, the full proceedings of which meeting are reported in A.E.C. Report No. TIC-7512; and book entitled "Applications of Ionizing Radiations" by Bernard Proctor, published 1952 by Cambridge Press, especially pages 4, 94, 95, 117, 168, 169, 198, 199, 200-203. Typical known types of neutron-producing devices include plasma focus devices such as are referred to in a paper of G. Decker et al entitled "Focus Devices With Respect to Density And Current Distribution, and Neutron and X-ray Emission" published in "Plasma Physics and Controlled Nuclear Fusion Research 1976, Vol. III, 6th Conference Proceedings, Berchtesgaden, 6-13 October 1976" by International Atomic Energy Agency, Vienna 1977, p. 441 ff., and other papers of that Conference.
One approach which has been attempted in an effort to obtain useful nuclear fusion reactions is disclosed in U.S. Pat. No. 3,489,645 of J. W. Daiber et al, issued Jan. 13, 1970. According to the method proposed therein, no magnetic field is utilized to increase the confinement time, and instead that patent proposes to produce such high temperatures and pressures in the plasma that the containment time becomes extremely short, for example on the order of 10.sup.-8 second. This is to be accomplished by first energizing a central region of a body of fusion material by applying a low-energy laser beam thereto to produce an "exploding" wave, and shortly thereafter impinging the periphery of the fuel with lasers of much higher energy, thereby to produce an "imploding" wave. It is asserted in the patent that very high plasma densities and very high temperatures are thereby produced, such that in about 10.sup.-8 second a useful fusion reaction occurs. So far as is known, the proposed method has not been scientifically or commercially successful, and it is apparent that in any event it requires extremely-precise aiming and timing of laser beams, as well as adjustment of their energy levels, in order to satisfy the conditions described in the patent.
U.S. Pat. No. 3,652,393 of Wolfgang Kaiser et al, issued Mar. 28, 1972 proposes to produce useful nuclear fusion by impingement of two opposed laser beams upon the inner opposed surfaces of a pair of semi-spherical fuel targets. The intention apparently is for the fuel material thereby spalled off of the interior opposed surfaces of the two targets to implode toward the center of the target segments, producing the desired plasma. So far as is known, this proposed method has not been scientifically or commercially successful.